Filter quality

All filters are not created equal

The most important consideration in purchasing a filter is its quality. Two new filters from different manufacturers may appear pretty much identical, but can be worlds apart in terms of their quality and their serviceability. There are cheap ways to make filters, and better ways, and it is important to know the differences.

LAMINATED FILTERS

Some colored filters are “laminated,” that is, made with two pieces of clear glass either sandwiched together with colored glue or that have a colored "gel" (gelatine film, which can be of excellent optical quality) placed between them. The glass can either be of optical quality or similar to the glass in a normal window.

One obvious characteristic of laminated filters is that they have four to six surfaces (because each piece that makes up the laminate has itself two surfaces), each of which can reflect light and affect the photographic image, particularly in backlit or sidelit situations. And if these surfaces are not perfectly flat and parallel to each other, the filter may act in the manner of a lens, altering the way in which light travels to the film. Over time, it's possible that the combined materials may de-laminate (separate) due to expansion and contraction. If this occurs, the filter may bubble, peal or become discolored, making it useless, except possibly as an experimental special effects filter that you may or may not ever use. The color of the gel may also shift or fade.

Colored gels can be used on their own as filters. Although many photographers tape them to the front of the lens, they can also be placed in “filter masks” – plastic or metal frames that hold gels – which are fitted to a filter holder that can be attached to a lens. They can also be attached in front of a light source, something that is frequently done in a studio. Gels don’t stand up well to moisture, heat or rough handling, and will fade over time. They must be kept in a dark, cool place, preferably protected between sheets of tissue paper. They are available in a huge variety of tones, and are relatively inexpensive to purchase.

ONE-PIECE FILTERS

Another manufacturing process produces a colored filter made from a single piece of optical glass that has had different pigments added to it while in a molten state to provide it with uniform color. The surfaces – there are only two, since it is a single piece of glass – are ground and polished so they are flat and parallel. Such filters won't fade, aren't subject to color shift, and of course can’t delaminate. They must be handled and stored carefully, though, and cleaned using a lens cloth, except that dust or grit should be blown off first.

One-piece filters made from plastics instead of glass can be relatively inexpensive, but are also susceptible to damage through normal handling. Their surfaces are delicate and can be easily scratched if placed in a pocket or even when cleaning. They don’t have the same quality rating as the better-made optical glass filters, but are quite serviceable for all but the most demanding photographers.

POLARIZERS MUST BE LAMINATED

All polarizing and circular polarizer filters must be made by a lamination process, in which a polarizing film is sandwiched between two layers of glass. They can't be made any other way.

COATING

Filters can be either coated or uncoated. Uncoated filters means both sides are simply bare glass which can reflect some light – as much as 9% – meaning that as little as 91% of the light striking the filter will go through it. The reflected light may also produce flare - stray light that can cause “ghosting” in the image. Ghosting occurs when a second faint image is reflected off the filter’s rear surface. Applying single layers of anti-reflective coating on both surfaces can cut filter reflection by half. Some manufacturers apply coating to only surface, generally the front side of the glass.

Coating is not applied to soft focus, fog, cross screen and prism filters since it won't improve their image quality.

MULTI-COATING

Multi-coated filters, with up to as many as five anti-reflective layers, are of sufficiently-high quality for the most exacting needs of professional and advanced amateur photographers, cutting reflection to as little as under 1%.

COATING METHODS

Quality filter coatings have staying power. Coatings that are bonded to the surfaces of the glass under extremely high heat are greatly resilient to wear and will provide years of consistent use. But, coatings that are “painted” on the filter’s surfaces or that are applied as a cold spray can wear off quite easily.

FILTER MOUNTS

Most circular filter frames (also known as mounts, rings or rims) are threaded to screw directly into the front of a lens. Such filters fit only lenses of the same diameter as the filter.

Some frames are made of plastic, which may cross-thread and jam if undue pressure is applied when attaching the filter. Quality filter frames are usually made of machined aluminum or brass. Aluminum has good durability and relatively-light weight, and is somewhat softer than brass. Some say brass is best, however manufacturers who use aluminum claim their filter frames will bend and absorb some of the shock if the front of a lens should accidentally strike something hard, thereby protecting the lens, whereas they say that a brass filter frame is so rigid that it will simply transmit the full force of the shock to the lens itself. Their point is that it's easier and cheaper to replace a filter than to repair a lens. Not a bad point. A potential downside to an aluminum rim is due to its relative softness, in that the filter may tend to jam or get stuck on the camera, making it difficult to remove. (You can always take off your shoe to remove a stuck filter. True, It works.)

FILTER SYSTEMS

Certain manufacturers make all their filters in only one standard width. They may be square, rectangular or circular in shape, and rimless. The filters slide into a filter-holder that can be attached to different-sized adapter rings made to fit all standard sizes of lenses. Photographers need only to buy one of each filter for use with all their lenses, provided they have the appropriate variety of adapter rings. It's a practical and economical concept. It keeps you from having to buy the same type of filter for each one of your lenses. Such systems are not only available for 35 mm cameras, but also for larger-sized formats.

The adapter ring is first screwed onto the lens, then the filter holder is snapped in place on the adapter ring. A square filter (about 3" by 3") can then be slid into one of two or three slots in the filter holder. Such systems can usually also accommodate round filters and may have a tighter slot for particularly narrow filters. The slots may be spring-loaded on one side so the filters don’t fall out when turned upside down. Because there is more than one slot, filters can be stacked in combinations for different effects. The filter holder (and therefore the filters) can usually be turned. There is even a “universal ring” available to fit non-standard lenses or lenses with damaged threads.

Filter selection for such systems is extensive - 75 and more different filters and masks - including many special effects filters (multi-image, blur, rainbow patterns, and so on). Masks block off a portion of the image and are used for such effects as double-exposure. Filters may be made of glass, gelatin film, plastics or vinyl-chloride. Glass filters are more resistant to scratching.

CONCLUSIONS

Manufacturers who use high-quality, one-piece optical glass and apply heat-bonded coatings supply the best filters in terms of both image quality and long-term serviceability. Such filters, when properly cared for, may in the long run prove to be less expensive than filters that will require periodic replacement. Unfortunately, price makes perfection, and the best filters can be considerably more expensive than their laminated or plastic counterparts. Most of us will detect differences between images taken with a superbly-crafted filter and an el-cheapo filter. The photographer who buys the best filter has one less thing to worry about.

Budget-conscious photographers who are experimenting with filters would be wise to purchase less-expensive filters, test them out and then consider purchasing high-quality filters when they know which ones they will use most.

A filter system is an inexpensive way to acquire single filters that can be used with all of your lenses.